1. Field of the Invention
The present invention relates to a fire-fighting system that drops flowable materials from a bucket carried by an aircraft, and more particularly, to such systems using simplified open-loop control to dispense such materials at a desired volume flow rate.
2. Description of Related Art
Large fires are often fought using aircraft to drop fire retardants, which can be specially formulated liquids, granular solid materials, or even plain water, either directly on the fire or at nearby locations to create a fire break. A very effective approach to this kind of fire fighting uses a large bucket suspended from a helicopter.
Helicopters are generally used for sustained fire fighting operations because they are more versatile than fixed wing aircraft. Since a helicopter does not require a landing strip, the bucket can be filled, without landing the helicopter, at a nearby body of water or at a temporary staging area set up to maintain a supply of fire retardant material close to the fire. This reduces the amount of time required between retardant drops, which likewise reduces the time required to extinguish the fire.
Buckets typically range in capacity from 90 to 3000 gallons or more. Many allow for partial filling to accommodate the lifting capacity of different helicopters with which they may be used. Known buckets come in a variety of configurations, a common one being a conic section with a tapered bottom. They may be made of a rigid material (often fiberglass to save weight) with a valved opening in the bottom, or they can be made of fabric mounted on a frame. They are slung from 25 to 200 feet under the helicopter, which hovers as the bucket is filled from either a nearby body of water or a tank filled with a flowable fire retardant material. The bucket or aircraft may have an on-board pump to reduce the amount of filling time. When the bucket is full, the helicopter proceeds to the drop zone where the fire retardant is emptied through an opening in the bucket, usually as the helicopter flies along a drop line.
While this is a highly effective manner of delivering large quantities of retardant to a fire burning over a large area, prior art implementations of this approach have numerous drawbacks.
For one thing, the fire retardant material must be deposited on the ground at a specific minimum concentration depending on the type of fire. Studies, such as Deeming, J. E., "The National Fire Danger Rating System--1978," General Technical Report INT-39, USDA Forest Service, Intermountain Forest and Range Experiment Station, Ogden Utah (1978), have set forth the concentrations of fire retardant needed on fires involving different types of vegetation.
Any concentration (volume flow rate) of retardant other than the optimum wastes valuable resources. A concentration that is too high obviously wastes retardant, as does a concentration that is too low. However, a more severe consequence is that an excessive retardant concentration reduces the area that can be covered by the fixed amount of retardant in the bucket. That increases the time required to extinguish the fire because more trips will have to be made to cover the area that is burning. An insufficient concentration over some of the drop area has the same effect because additional depositions will have to be made at locations that did not receive enough retardant. Not only does increasing the time to extinguish the fire cause additional loss of whatever is in the path of the fire, it also requires additional aircraft operating time. This is especially critical considering that fire-fighting aircraft and operating crews are often paid thousands of dollars per hour. If the additional time requires the use of additional helicopter crews, the cost increases further still.
The goal, then, is delivery of no more or less than the desired coverage level along a particular drop line. Even better would be the ability to control drop line length.
The volume rate of flow of the material from the bucket affects the actual deposition rate on the ground more than any other factor. The most basic manner of emptying the bucket carrying the retardant simply opens a valve as quickly as possible when the helicopter approaches the drop line, thus allowing the material to flow uncontrolled out of the bucket. There are many different types of valves, both electrically and hydraulically driven, known to be suitable to this purpose. A fabric bucket can use flaps that are sealed by the pressure head of the retardant and opened by pulling a rope attached to the flaps. In any case, simply opening the bucket and letting the fire retardant material empty out results in a varying deposition rate as the bucket empties.
U.S. Pat. No. 5,279,481, U.S. Pat. No. 5,320,185 and U.S. Pat. No. 5,451,016 address the problem of how to achieve a constant volume flow rate of retardant from a bucket.
The approach in these patents monitors the liquid level in the bucket throughout deposition and uses closed-loop control circuitry to match the volume flow rate out of the bucket to the desired rate. These systems are very effective in providing the necessary control, but the precision they afford is simply not needed in this operating environment. That is, considering that the volume flow rates are typically hundreds of gallons per second, and that the retardant is being dropped from a height of a hundred or so feet, complex closed-loop control systems are not very cost effective because the degree of control they provide is well in excess of what is necessary to provide effective fire fighting capability.
There are also other problems with such an approach. Monitoring the fluid level or the volume of the retardant as the bucket empties is straightforward in theory, but in reality the bucket is buffeted by external forces that cause the retardant to slosh around and make it difficult to obtain an accurate reading of the amount of retardant left in the bucket at any particular time. U.S. Pat. No. 5,320,185 and U.S. Pat. No. 5,451,016 disclose a way of electronically compensating for this effect, but that simply increases the complexity of the control system. In addition, these systems require electronic control circuitry to be in constant communication with sensors in the bucket monitoring the retardant level. The inevitable rough handling of the bucket in this operating environment will contribute to breakdowns of such a system, either in the sensors, the control circuitry, or the cables over which they communicate. That results both in lost operating time for the bucket and increased cost attributable to expensive replacement parts.
Unfortunately, up to now there has been no other known manner of providing the desired degree of control over the retardant volume flow rate.